The present invention relates to the installation of patches to seal holes in pipes generally, and more particularly to a method and apparatus for sealing perforations in steam or water injection oil wells.
Oil wells may be generally classified as being of one of three different types; production wells, steam injection wells, and water injections wells. Production wells are used to pump oil from an oil bearing formation. Steam injection wells serve to heat certain types of particularly thick oil and thereby aid production wells in extracting oil. Water injection wells operate on the principle that oil floats on water and therefore oil extraction may be aided by pumping water below an oil pool and thereby raising the level of the pool. Oil wells are typically made by drilling a borehole in an oil bearing formation. The borehole is typically about six to twenty inches in diameter and may range from about 400 to about 20,000 feet in depth. After drilling the borehole to a desired depth, a steel pipe or casing is installed in the borehole from the surface to the bottom of the hole. Such casings vary in diameter depending upon the application. Casings of three and one-half inches in outside diameter are one common size for steam injection wells. After installation of the pipe, cement is typically pumped down the pipe under substantial pressure so that the cement will exit the pipe at the bottom of the borehole and will flow to the surface, thereby filling the gap between the outside of the pipe and earthen inner wall of the borehole. Upon completion of the process an open pipe encased in cement runs from the surface to the bottom of the original earthen borehole.
In many oil bearing formations, the oil is very thick having a consistency similar to that of molasses. Oil of this type cannot effectively be pumped to the surface without first heating the oil to reduce its viscosity to a point where the oil is in a flowable state. It has long been standard practice in the oil industry to heat such thick oil by means of steam injection. In a steam injection oil well, perforations, for the injection of steam into the oil are created at predetermined intervals along that portion of the well casing that passes through the oil bearing formation. For example, if the last two hundred feet of an 1800 foot wellbore pass through an oil pool, four perforations will typically be made at 50 foot intervals along the casing. Typically, the perforations are about one quarter of an inch in diameter and are formed by xe2x80x9cshapedxe2x80x9d explosive charges which are lowered to the desired depths in the well. The shaped charges are capable of penetrating both the inner steel well casing and the outer concrete casing.
A common problem in steam injection oil wells is that as the oil is pumped from the well, the oil level drops and eventually uncovers the perforations. When this occurs steam enters the evacuated region formerly occupied by oil and forms what is known in the industry as a xe2x80x9csteam chest.xe2x80x9d The loss of steam through uncovered perforations greatly reduces the efficiency of the well and in many cases where multiple perforations are uncovered the recovery of the remaining oil reachable by the well becomes cost prohibitive. In order to keep such wells productive, attempts have been made to develop methods of sealing the uncovered perforations. Perhaps the most common prior art approach to sealing perforations has been to form a sealing sleeve, where the sleeve comprises a cylindrical steel portion with rubber-like gasket material bonded to the outer surface of the steel sleeve. The sleeve and gasket typically have a combined thickness of about 0.5 inch. Thus, when deployed the sleeve creates a significant restriction in the diameter of a typical wellbore. Generally, for deployment, the sleeve is wrapped about an expansion device which is typically a mechanically operated expander plug and is subsequently lowered to the uncovered perforation. Upon reaching the perforation the sleeve is expanded to seal the perforation. Typically, the sleeve is held in place in the well casing by friction. Devices similar to the one described above except that an explosive charge is substituted for the expander plug have also been tried.
The prior art approaches suffer from several drawbacks, with the foremost being the reduction in the inside diameter of the wellbore in the region of the sealing sleeve. Such a reduction in diameter severely limits the types of downhole tools that may subsequently be deployed in the bore. In addition, since prior art sealing sleeves are wrapped about an expansion device, there is typically only a small amount of clearance between the outside diameter of the sealing sleeve and the inside diameter of the wellbore. As a consequence, any irregularity in the casing due to deposits or a slight bend in the casing will cause a typical prior art sealing patch to xe2x80x9chang upxe2x80x9d or become wedged in the wellbore. Also, prior art sealing sleeves have been known to leak severely and, on rare occasions, slip in the bore and consequently uncover the perforation they were intended to seal. Further, the prior art sealing methods have proven to be relatively costly to employ.
What is needed therefore is a sealing device and method that can effectively seal a casing perforation with a relatively thin patch that does not significantly reduce the size of the borehole. Another highly desirable feature would be the ability to place a subsequent patch at a location below an existing patch. Preferably, such a device and method will also provide a mechanism for more securely adhering the patch to the wall of the wellbore, and will further be of relatively low cost to produce and deploy.
The present invention casing patching tool overcomes many of the problems of the prior art by providing an apparatus and method for delivering a thin spirally wound or coiled patch possessing spring-like properties that effectively seals wellbore perforations without significantly reducing the bore""s internal diameter. The patch is preferably formed from spring-steel and uses spring tension to increase adherence to the wall of the wellbore. The invention provides the ability to place a subsequent patch at a location below an existing patch. Consequently each perforation in a steam injection well may be sealed as the level of oil in the formation drops and thereby exposes the next lower perforation. Further, the present invention casing patching tool may be fabricated at relatively low cost and is comparatively easy to deploy and use.
The primary components of the invention preferably comprise a spirally wound metal patch, a patch delivery tool, and a patch rolling tool. The delivery tool further comprises a barrel and a plunger assembly, where the plunger assembly is slidably received within the barrel. Prior to use, the patch is wound into the form of a cylinder by means of the rolling tool and is loaded into the barrel of the delivery tool. To prevent inadvertent deployment of the patch while lowering the delivery tool to the location of a perforation, a shear pin is used to hold the plunger assembly in a fixed position with respect to the barrel. To deploy the patch, hydraulic pressure is applied to the plunger assembly causing the barrel to shear the shear pin and move upwardly with respect to the stationary plunger. The patch abuts the plunger and remains stationary while the barrel travels up the pull tube. At the end of its travel, the barrel will have slid entirely off the patch, thus freeing the patch within the wellbore. Upon deployment, the patch unwinds within the wellbore and seals the perforation in the casing wall. Spring tension tends to keep the patch securely fixed over the perforation. These and other features of the invention will become more apparent from the following detailed description of the invention, when taken in conjunction with the accompanying exemplary drawings.